Place the oven rack in the center position and preheat your oven to 350°F (176°F). Lightly grease a 9” x 13” glass baking dish or cast iron skillet with butter or cooking spray.
Toss 5 cups blueberries, the zest and juice of 1 lemon, ¼ cup granulated sugar, 2 tablespoons all-purpose flour, ¼ teaspoon ground cinnamon, and ⅛ teaspoon ground nutmeg in a mixing bowl. Spread the mixture evenly in the baking dish. Distribute 2 tablespoons unsalted butter, cubed on top.
Whisk together 1 ½ cups all-purpose flour, ¾ cup granulated sugar, 1 teaspoon baking powder, and ½ teaspoon fine sea salt in a mixing bowl. Add 6 tablespoons unsalted butter, melted, ⅓ cup sourdough discard, and 1 teaspoon vanilla extract and mix until it resembles a crumbly cookie dough consistency. Use your hands to crumble it evenly over the blueberry filling.
Bake the cobbler for 40-45 minutes or until the fruit is bubbling around the edges and the top is golden brown. If you are using frozen blueberries, allow a few extra minutes of baking time. Let the cobbler cool for 30 minutes before serving to allow the filling to set.